The use of morphine and related opioids both at the clinical and recreational level is characterized by the development of tolerance and/or dependence. Tolerance occurs in the nociceptive modulatory system, which includes the periaqueductal grey (PAG) to the spinal cord. The cellular adaptations that occur during opiate tolerance limit the effectiveness of opiates as treatment for severe pain. Mu-opioid receptor (MOR) tolerance has been hypothesized to develop at the level of the receptor. Our recent evidence indicates that tolerance is caused by an increase in receptor sensitivity to morphine. The cellular changes that lead to the changes in MOR signaling are not known, but need to be examined to more fully understand morphine tolerance. MOR agonists activate multiple signaling pathways, including the activation of Mitogen Activating Protein Kinase (MAPK) pathways. If the sensitivity of the MOR is increased with chronic administration of morphine, then morphine should cause ERK1/2 phosphorylation. If ERK1/2 signaling contributes to morphine tolerance, then ERK1/2 phosphorylation should be greater in tolerant rats. However, we have found that co-administration of the MAPK/ERK kinase (MEK) inhibitor U0126 with chronic morphine treatment enhances morphine tolerance. This finding suggests that activation of the ERK1/2 pathway does not facilitate MOR desensitization, but blocks morphine tolerance. The studies in this proposal will address this controversy by determining the role of ERK1/2 in morphine tolerance. This will be accomplished by determining how stimulation of the MOR alters ERK1/2 signaling and how disruption of ERK1/2 signaling affects the development of morphine tolerance. Relevance: Tolerance is a serious medical problem contributing to the suffering of chronic pain patients. The role of ERK1/2 signaling during the development of tolerance is a potentially important and relatively unexplored signaling pathway that may contribute to tolerance. Opiate tolerance involves the development of adaptive changes in MOR and its associated pathways leads to neuronal plasticity in the brain. The adaptations in MAPK signaling may directly impact neuronal plasticity associated with opiate tolerance in humans. The current study will determine the role of ERK1/2 signaling during the development of tolerance. If ERK1/2 activation blocks tolerance as expected, it may be possible to develop treatments that enhance ERK1/2 activation.